1. Field of the Invention
The present invention relates to a process for producing finely divided porous polymer particles, particularly, to a process for producing finely divided porous polymer particles composed of a copolymer of a carboxyl group containing vinyl compound and another vinyl compound.
2. Description of the Prior Art
Hitherto, in the field of photography, silica particles or insoluble synthetic polymers have been used in top layers or back layers as matting agents to prevent adhesion or the occurrence of electrostatic charges. For example, when photographic materials are stored or packed in the rolled state, they have the tendency to deteriorate in preservability because of adhesion therebetween. Therefore, a matting agent is used to prevent such adhesion. The surface of the photographic material becomes coarse due to the use of the matting agent. This coarseness prevents the occurrence of electrostatic charges, because the area of contact of the photographic material becomes relatively small. Further, the formation of Newton's rings during printing or enlargement can be prevented by the matting agent.
Moreover, the incorporation of silica particles or finely divided insoluble synthetic polymer particles into a sensitive silver halide emulsion layer of the photographic material brings about the effect that abrasion during drying is remarkably decreased and the occurrence of scratches during storage or packing of the photographic material is prevented.
However, there are many defects when silica particles or insoluble synthetic polymer particles are added to a protective layer for a sensitive silver halide emulsion layer of a photographic material. For example, when silica particles or insoluble synthetic polymer particles exist in a hydrophilic colloid protective top layer for a photographic material, the photographic material has, though slightly, a milky appearance which is not desirable for transparent photographic material. Further, these finely divided particles do not remain in only the top layer but move into the sensitive silver halide emulsion layer to displace the silver halide emulsion, and, consequently, pin-holes sometimes occur after development, that is, spots where particles of the matting agent were present become white.
It has been described in U.S. Pat. Nos. 2,992,101 and 3,767,448 that finely-divided polymer particles which are soluble a developing solution and are composed of a 1:1 copolymer of a carboxyl group containing vinyl compound, such as methacrylic acid, and an alkyl methacrylate, such as methyl methacrylate, are incorporated into a top layer or back layer of a photographic material. According to this process, the occurrence of a milky appearance decreases because the finely divided polymer particles dissolve during development of the photographic sensitive material. However, if the particle size of the soluble polymer particles is too large, it is difficult to prevent the pin-holes, that is, spots where the polymer particles were present become white. Further, in this case, it is impossible to completely avoid the occurrence of the milky appearance, because dissolution of the polymer particles during development is not perfect.